1. Field of the Invention
This invention relates to a modified process for the liquid phase synthesis of cellulose carbamate using more effective activators of the initial pulps, which process has utility in the manufacture of cellulose fibers, films, and other products.
2. Summary of the Related Art
For the production of regenerated cellulose products, it is known to use cellulose carbamate as a soluble cellulose compound instead of cellulose xanthate, whose use is problematic because of its toxicity and flammability and because of stringent environmental regulations.
Cellulose carbamate is obtained by heating a homogeneous mixture of cellulose and urea at a temperature above 110.degree. C. either in the absence of a liquid medium (U.S. Pat. No. 2,134,825, EP-Patent 57 105, EP-Patent Application 402 707) or in the presence of an inert organic liquid (EP-Patent 97 685, U.S. Pat. No. 5,378,827). In the solid-phase process, the cellulose/urea mixture may contain 3 to 50 wt-% of biuret (DE-Patent 35 08 611). (As used herein, all values of wt % are based on cellulose.) However, these well-known methods for the manufacture of cellulose carbamate, both in solid or liquid phase, suffer from the disadvantage of having an insufficiently uniform distribution and substitution of carbamate groups in the cellulose chain, which negatively affects the solubility behavior, the solution properties, and the spinnability of the cellulose carbamate, as well as the properties of the regenerated products. Furthermore, the distribution of urea intercalated into the cellulose structure is not sufficiently uniform, which negatively affects the cellulose carbamate properties, especially its solubility as well as the stability and spinnability of its solution.
For the dry-phase cellulose carbamate process, several methods of preliminary activation of the cellulose pulp have been proposed, for example with ammonia (Polish Patent 160 863), with inorganic or organic salts (U.S. Pat. No. 2,134,825, Polish Patents 160 866 and 163 049), with alkalihydroxide (U.S. Pat. No. 2,134,825), which is thereafter washed-out (EP-Patent 178 292) and/or neutralized by an acid (EP-Patents 402 605 and 402 707) such as carbonic acid (Polish Patent 165 916), with water at 100 to 170.degree. C. and hyperbaric pressure (DD-Patent 298 789), or with enzymes and optionally thereafter ammonia (Polish Patent 159 085).
None of these known activation methods apply to cellulose, which is thereafter converted in the liquid phase. The solubility and spinnability of cellulose carbamate obtained by a liquid-phase process may, however, be improved by treating the carbamate with diluted aqueous acid, like 1N hydrochloric acid at a temperature above 100.degree. C. under autogeneous pressure (DE-Patent 44 43 547).
All the well-known methods for the manufacture of cellulose carbamate fail to use sufficient effective activators of cellulose pulps to obtain all the properties of cellulose carbamate required to reach fiber and foil-grade level of this product. Accordingly, new and improved methods of producing cellulose carbamate are desirable.